$\begingroup$I've once heard that 60 km/h is the switching point, above which air conditioning is more efficient, but I have no further prove.$\endgroup$
– BernhardJul 21 '12 at 17:28

$\begingroup$Interesting. That figure agrees with my driving around the local back-roads and watching the consumption, but I'm not sure I'd enjoy the drive from the south up to Madrid at that speed! ;-)$\endgroup$
– Robin WilliamsJul 21 '12 at 17:58

$\begingroup$No, then you hit the gas, close the windows and turn on airconditioning. But I think the outside temperatur is also an important factor$\endgroup$
– BernhardJul 21 '12 at 18:02

2 Answers
2

That is a rather difficult calculation, even if we assume that both processes are equally effective at cooling. You would need to know the drag coefficient of the car with windows up and down as well as the efficiency of the AC. The best way to settle a question of this sort is with an empirical experiment.

It turns out that this is precisely what the Mythbusters did, as reported in this post. They found that driving with the windows down significantly beat driving with the AC at full blast, getting 24 kilometers more out of the windows down car. They were driving at 72 kph, which is a little slower than your case.

Let´s assume... it´s not valid at all. It depends completelly on many factors: interior T, exterior T, performance of your AC equipment, your "feeling"... From my point of view, of course you can reach the outside T of the car, which could be -10°C if you want (in winter)... and your AC won´t be able to reach that T for sure.

But in summer, with 30°C outside, there´s no way for you to have better results by opening the windows: you need to turn on your AC, so spend some money in gas, if you want to have less than 30°C in your car.

For your information, cars are equipped with AC capable to reach a confort T (let´s say around 25°C in heads). But that´s a little bit tricky too: Many people prefers to "feel" more than that or more, so they control the AC in different ways. You can only control the epperture of windows, more o less, but you´ll never be below the out T.
Just an opinion, but your question and assumption is kind of lazy. Best regards,

$\begingroup$how cool we feel doesn't depend upon temperature alone. We feel cool when we lose energy to the outside environment. If the outside temperature is 30 degrees, our body is 37 degrees and there is a strong wind passing us, then there is more air in contact with the body, which can steal the energy from our body. If however there is no outside air, we will heat up the air around our bodies, and it will take longer for this hot air to be replaced by cooler air. Thus we will feel colder in the wind, despite both scenarios being 30 degrees.$\endgroup$
– KenshinApr 27 '13 at 8:32

$\begingroup$Its not about temperature, its about heat loss. Heat loss is greater in wind. Temperature affects heat loss but its not the only factor.$\endgroup$
– eJuniorAug 25 '13 at 14:22

1

$\begingroup$This is no answer at all. The OP specifically said to assume both methods are acceptable to the occupant. The real question is therefore which burns more gas, running the air conditioner or rolling down the windows? You didn't address that at all.$\endgroup$
– Olin LathropNov 23 '13 at 16:44